Temperature control system with multiple thermostats

ABSTRACT

A heating-cooling control system includes thermostats in the discharge and return air passages, and a temperature set dial. A selection circuit energizes one of the thermostats below a preset temperature, and the other thermostat is energized above that temperature. A heat override circuit prevents a call for heat above a certain temperature. With two compressors, the first compressor is maintained off by a timer circuit for a short time to prevent cycling of the compressor. Another timing circuit briefly delays operation of the second compressor, when the cooling system calls for its operation, to prevent simultaneous energization of both compressors and an excessive demand on the electrical supply line or unit.

TEMPERATURE CONTROL SYSTEM WITH MULTIPLE THERMOSTATS 1 Get. 30, 19733,500,898 3/1970 Gerhart, Jr. et al. 165/26 Primary Examiner-Manuel A.Antonakas [75] Inventor. Frank E. Wills, York, Pa. Att0mey Donald wBanner at a]. [73] Assignee: Borg-Warner Corporation, Chicago,

57 ABSTRACT [22] Filed: July 12, 1972 A heating-cooling control systemincludes thermostats [21] Appl No: 271,087 in the discharge and returnair passages, and a temperature set dial. A selection clrcuit energizesone of the thermostats below a preset temperature, and the other [52]U.S.Cl 165/27, 236/91, 307/39 thermostat is energized above thattemperature. A [51] Int. Cl..... F251) 29/00 heat override circuitprevents a call for heat above a [58] Field 01 Search 165/14, 22, 26,27; certain temperature.

236/91 91 F; 307/39 With two compressors, the first compressor ismaintained off by a timer circuit for a short time to [56] ReferencesC'ted prevent cycling of the compressor. Another timing UNITED STATESPATENTS circuit briefly delays operation of the second 3,612,165 10/1971Haynes 165/26 compressor, when the cooling system calls for its2,689,932 9/1954 Homfeck 236/91 operation, to prevent simultaneousenergization of Ct both compressors and an excessive demand on the3,144,991 8/1964 Marchant... 236/91 electrical Supply line or unit3,616,846 11/1971 Wills 165/26 3,292,689 12/1966 Evans 165/26 6 Claims,3 Drawing Figures THERMOSTATS TEMP RETURN DISCHARGE SET IO ll l2COMPARATOR 1 14 23 24 L COOLING 1a 20 j NREGULATING I6 22 SIGNALREFERENCE SELECTION sasmu. 1 #25 17 13 I15 31 3O REFERENCE /.a 33

SIGNAL HLAT 32 HEATING OVERRIDE PATENTEDUU 30 I975. I 3.768545 SHEET 28F3 I RETURN DISCHARGE l4 Wser W'W i W FIGZ ' SHEET 0F 3 TO HOT GASCONTROL T0 2 as (2%: 8 I05 I06 I07 COM 97 95 9e HEAT CONTROL TEMPERATURECONTROL SYSTEM WITH MULTIPLE THERMOSTATS BACKGROUND OF THE INVENTIONHeating-cooling systems which operate over considerable load variationswith multiple compressors have been developed for some time. There arestill problems associated with such systems. One problem is with thecooling of fresh produce, such as lettuce, carried in railroad box cars.In general such systems utilize a single thermostat sensing temperaturebelow the top of the produce mass, so that when this sensed temperatureindicates suitable cooling of the load, the top layer of the produce isfrequently frozen. This frozen material is wasted and must be discarded.

Another problem frequently encountered is the cycling on and off ofclosely controlled equipment as the sensed temperature varies up anddown, just above and below the control point. In a multi-compressorcooling syste, it can happen that more than one compressor is brought onthe line simultaneously by a high cooling demand, causing an overload ofthe electrical supply circuit or equipment.

It is a principal consideration of this invention to provide an improvedheating-cooling system which obviates the freezing and destruction ofthe top layer of produce, when perishables are cooled by the system.

Another important consideration of the invention is to provide amultiple compressor system in which the first compressor energized isnot cycled on and off, but delayed in its re-energization, to obviateexcess cycling.

Still another important consideration of the invention is the provisionof a system which regulates energization and de-energization of thefirst compressor, loading and unloading of the first compressor,energization and deenergization of a second compressor, and secondcompressor loading and unloading, in the appropriate sequence tominimize excessive drain in the electrical supply circuit.

SUMMARY OF THE INVENTION A control arrangement constructed in accordancewith this invention is useful to regulate operation of a cooling systemwhich discharges air into a space and receives return'air from thatspace. The control arrangement comprises a first thermostat disposed toprovide a first temperature signal indicative of the discharge airtemperature, and a second thermostat disposed to provide a secondtemperature signal indicative of the return air temperature. Anadjustable temperature set unit provides a set signal indicating thedesired temperature of the space.

In accordance with one aspect of the invention, a selection circuit isconnected to receive both the set signal and a first reference signal.This selection circuit completes an operating circuit for the firstthermostat when the value of the set signal is less than the value ofthe first reference signal, and completes an operating circuit for thesecond thermostat when the value of the set signal is greater than thevalue of the first reference signal. A comparator circuit is connectedto receive both the set signal and the temperature signal from theenergized thermostat, and to provide a regulating signal, related to thedifference between the set signal and the temperature signal from theenergized thermostat, to regulate associated heating and/or coolingequipment.

LII

In addition such an arrangement, when used with at least twocompressors, includes a first time-delay circuit to preventre-energization of the first compressor for a minimum time after theregulating signal decreases and the first cooling control stageindicates the first compressor should be taken off the line. A secondtime-delay circuit, coupled to the second cooling control stage whichgoverns the second compressor, delays the energization of the secondcompressor when the regulating signal indicates a call for additionalcooling, to prevent simultaneous energization of the compressors and aconsequent high demand upon the electrical supply circuit.

THE DRAWINGS In the several figures of the drawings, like referencenumerals identify like components, and in the drawings:

FIG. 1 is a block diagram, partly in schematic form, depicting thegeneral arrangement of a portion of this invention;

FIG. 2 is a schematic diagram which sets out circuit details ofcomponents shown generally in FIG. 1; and

FIG. 3 is a schematic diagram which depicts the remainder of the controlarrangement of the invention.

GENERAL SYSTEM DESCRIPTION FIG. 1 shows a general system layout of aportion of the control arrangement of this invention. Conventionalequipment, such as a compressor and hot gas conduits utilized inregulating the cooling, and coils or other units for effecting theheating, are not depicted. Similarly the actual discharge and return airvents or conduits are not shown. Such components are well known and canbe located in many environments, such as a railroad box car, truck,stationary enclosure, or any other space to be cooled and/or heated bythis systern.

FIG. 1 shows a first thermostat 11 disposed to provide a firsttemperature signal which indicates the discharge air temperature. Asecond thermostat 10 is positioned to provide a second temperaturesignal which indicates the return air temperature. Both temperaturesignals do not appear on line 12 simultaneously, because selectioncircuit 13 completes an operating circuit for only one of the first orsecond thermostats, so that only one temperature indicating signalappears on line 12 at any one time. A temperature set unit 14, which canbe a simple potentiometer or other device, provides a temperature setsignal on line 15 and, over line 16, to selection circuit 13. Theselection circuit also receives a first reference signal over line 17.When the value of the set signal is less than the value of the firstreference signal, an operating circuit is completed over line 20 forfirst thermostat 11. When the value of the set signal exceeds the valueof the first reference signal, an operating circuit is completed overline 18 for return air thermostat 10. This arrangement facilitates theestablishment of a crossover point or reference temperature by the valueof the reference signal passed over line 17 to selection circuit 13. Forexample this reference signal can be set to represent a temperature of32F. Then if the temperature set unit 14 is adjusted to call for atemperature above 32, selection circuit l3 completes an energizingcircuit over line 18 for return air thermostat l0, and allows thisthermostat to do the controlling. When the temperature set unit 14 isadjusted to call for a temperature below the crossover point indicatedby the signal on line 17, then the energizing signal is provided on line20 so that the discharge air thermostat 11 does the controlling.

Comparator circuit 21 is connected to receive both the set signal, overlines and 22, and the temperature signal, over line 12, from whicheverone of thermostats 10, 11 is then energized by selection circuit 13. Thecomparator stage provides an output or regulating signal on line 23which is related to the difference between the set signal on line 22 andthe temperature signal on line 12. This regulating signal is passed overline 24 and common line 25 to the cooling control stages, to bedescribed below in connection with FIG. 3. The regulating signal is alsopassed from line 25 over resistors 26 and 27, and line 28 to the heatingcontrol circuitry, which will also be described in connection with FIG.3.

In accordance with another aspect of this invention a heat overridecircuit 30 is provided, and has one input connection which receives asecond reference signal over line 31. This second reference signalisusually of a different value then the level of the first referencesignal supplied over line 17 to the selection circuit. The level of thesecond reference signal establishes a base against which the temperatureset signal, also received by override circuit 30 from line 15, iscompared to provide a lock-out signal on output conductor 32. Forexample, the amplitude of the reference signal on line 31 can beselected to represent a temperature of F. When temperature set unit 14is adjusted to call for a temperature below 20, or below any otherreference level established by the signal on line 31, an output signalis provided on line 32 and passed over diode 33 to conductor 28, tooverride the regulating signal from line 25. With this generalperspective, a more detailed description of the invention will now beset out.

DETAILED DESCRIPTION OF THE INVENTION In the left hand portion of FIG. 2a voltage divider arrangement 35 is shown connected between anenergizing conductor 36 and a plane of reference potential, commonlydesignated ground. Resistors 37-43 are connected in seriesbetweenconductor 36 and ground to establish different voltage levels orreference'signals, utilized to signify different temperature levelsagainst which the actual temperature signals and the set point signalcan be compared, and against which the regulating signal applied to thecooling and heating control stages can be referenced. Conductors 44-50,inclusive, are individually connected to different circuit points in thevoltage divider 35 as shown to establish voltage levels, or referencesignals, connoting different temperature levels. Another reference unitor voltage divider arrangement 51 includes resistors 52, 53 and 54connected in series between conductors 47 and 50. Conductor 17 isconnected to the junction of resistors 53 and 54, and is also coupledover resistor 55 to one input connection of an operational amplifier 56,connected as a voltage level detector in the thermostat selectioncircuit 13. Temperature set unit 14 is a potentiometer having its endconnections coupled to conductors 47 and 50. The other input connectionof op amp 56 receives the temperature set signal from the movable tap ofpotentiometer 14, over conductor 15, resistor 57 and conductor 16. Thepin 4 connection of this op amp is grounded, and its 8 pin connection iscoupled to conductor 36, or B+. This is also true (B+ on pin 8, and pin4 grounded) for the other op amps 21 and 30 in FIG. 2, and for thecontrol stage op amps 88 and in FIG. 3. All these units are connected asSchmitt trigger circuits. It will be understood that although differentphysical conductors are actually used for each stage, the same referencenumeral 36 is employed to indicate the connection of the number 8terminal to B+.

Selection circuit 13 also includes three NPN type transistors 58 60 and61. The output connection of op amp 56 is coupled over a resistor 62 toinput connection 3, and the output connection is also coupled toconductor 63. The base of transistor 58 is coupled to the commonconnection between resistors 64 and 65, which resistors are coupled inseries between conductor 63 and ground. The emitter of transistor 58 isgrounded and its collector is coupled to the common connection betweenresistor 66 and the anode of diode 67. The other end of resistor 66 iscoupled to energizing conductor 36, and the cathode of diode 67 iscoupled to the base of transistor 61. The base of transistor 60 iscoupled over diode 68 and resistor 70 to conductor 63. The emitters oftransistors 60, 61 are both connected to conductor 50, and thecollectors of these two transistors are respectively coupled overconductors 18, 20 to the return and discharge thermostats 10, 11. Thesymbol T on these thermostats indicate a Thermistor is employed in thepreferred embodiment of this circuit. The other connection of eachThermistor is coupled to conductor 12, and a resistor 71 is connectedbetween conductors 12 and 47 to provide a return circuit for whicheverone of Thermistors l0, 1 l is energized by circuit 13. I

In circuit 13, let it be assumed that the value of the first referencesignal on line 17 represents a temperature of 32F. It is further assumedthat the temperature called for by set unit 14 is below 32, and underthese conditions it is desired to regulate the system with an outputsignal from discharge air thermostat 11. At this time the output signalfrom stage 56 is low, and there is no gate drive to transistor 60 tocomplete an operating circuit for Thermistor 10. However current flowsfrom energizing conductor 36 over resistor 66 and diode 67 to the baseof the transistor 61, driving this transistor on and completing anoperating circuit'for the discharge air thermostat 11. Thus under theseconditions the temperature signal from Thermistor 11 is passed over line12 and resistor 72 to the 6 input connection of comparator circuit 21.

Assuming now that the temperature called for by unit 14 is changed, andis now above 32" or whatever reference level is represented by thesignal on line 17, then the set signal on line 15 is effective overresistor 57 and conductor 16, to cause amplifier 56 to switch, and theoutput of this amplifier goes high. This provides gate driver overconductor 63, resistor 70 and diode 68 to the base of transistor 60,turning this transistor on to complete an energizing circuit for returnair thermostat 10 over line 18. At the same time, as the signal onconductor 63 goes high, this provides a gate drive signal over thevoltage divider circuit 64, 65 for transistor 58, which is driven on. Astransistor 58 conducts it provides a virtual ground at the anode ofdiode 67, which robs transistor 61 of the gate drive previouslyreceived. Those skilled in the art will appreciate that the transitionin control, from one thermostat to the other, can be achieved at anydesired temperature by changing the level of the first reference signalprovided on line 17.

Comparator circuit 21 receives the set signal over conductor 15,resistor 73 and conductor 22 at its 5 input terminal. This comparatorcircuit also receives the temperature-indicating signal, over line 12and resistor 72, from the energized Thermistor. The output connection 7of this stage is coupled over resistor 74 to a common connection 75. Aregulating signal is provided from point 75 over conductor 24 to theassociated control circuits to be described in connection with FIG. 3. AZener Diode 76 is coupled between connection 75 and ground. Capacitor 77is coupled in parallel with resistor 78, and this parallel circuit iscoupled between terminal 75 and input connection 6 of comparator stage21. The comparator stage operates in a well known manner to provide theregulating signal on conductor 24 and common line 25 as a function ofthe difference between the set signal received at input connection 5 andthe temperature signal received at its other input connection 6.

At the bottom of FIG. 2 heat override stage 30 also comprises an op amp.Its 6 input connection is coupled over a resistor 80 to conductor 31,over which the second reference signal is received. The other inputconnection 5 is coupled over a resistor 81 to conductor 15,

over which the set signal is received. A capacitor 82 is coupled betweenthe two input terminals, and a resistor 83 is coupled between the 5input connection and the output connection 7, which output connection isalso coupled to line 32. This stage 30 functions as the heat overridecircuit, to provide a lock-nut signal on line 32 to prevent operation ofthe heat equipment when the temperature called for by the set signal online is below the value of the temperature represented by the secondreference signal on line 31.

Considering now the circuitry depicted in FIG. 3, the regulating signalis received over line 24 and distributed over common conductor 25. Inaddition the signal from heat override stage 30 is received overconductor 32. Conductors 44-49, inclusive, are connected as alreadydescribed to the difierent voltage points in the voltage divider-orreference arrangement 35, to pro-' vide different reference signals tothe control stages in FIG. 3.

The op amps 85, 86, 87 and 88 are connected as cooling control stages tooperate or switch in succession and provide actuating output signals tobring on additional cooling as the level of the regulating signal oncommon line 25 first exceeds that on line 48, then exceeds the level ofthe reference signal on line 46, next exceeds the reference signal online 45, and finally is greater than the level of the reference signalestablished on conductor 44. The other op amp, 90, is a heating controlstage used to provide an actuating output signal to associated heatingequipment to bring on the heat. In that there is only one heat controlstage, this will first be described.

A capacitor 91 is coupled between energizing conductor 36 and the commonconnection between resistors 26, 27 in the input circuit of op amp 90.The other input connection 3 of this stage is coupled over resistor 92and conductor 49 to the voltage divider arrangement, to receive areference signal as already described. A capacitor 93 is connectedbetween the input terminals, and a resistor 94 is coupled between theoutput terminal 1 and the input connection 3 of this op amp.

Absent any override signa on line 32, the regulating signal on line 25is applied to the input terminal 2, and the reference signal fromconductor 49 is passed to the other input connection 3. When the levelof the regulating signal exceeds that of the reference signal, op ampswitches and provides a higher or positive output signal at connection1, which is passed over resistor 95 to rapidly drive on NPN typetransistor 96. Conduction of transistor 96 provides an output signal onconductor 97 to actuate the associated heating equipment. For example,this signal can operate a heat controlling relay to pass electricalcurrent through heating coils and provide heat to the enclosed spaceregulated by the control system of this invention. In the circuit oftransistor 96, diode 98 is connected between its collector and emitteras shown, and resistor 100 is coupled between its base and emitter. Theemitter, one end of resistor 100, and the anode of diode 98 areconnected to ground.

Considering now the first cooling control stage which includes op amp85, the regulating signal on common line 25 is passed over resistor 101to the 3 input connection of this stage. Capacitor 102 is coupledbetween the input connections. The reference signal supplied over line48 is passed over resistor 103 to input connection 2. The outputterminal 1 of this op amp is coupled over resistor 104 to inputconnection 3.

When the amplitude of the regulating signal on common line 25 passedover resistor 101 to terminal 3 exceeds the value of the referencesignal present at input terminal 2, op amp 85 switches to provide apositive or high actuating output signal on conductor 105, which signalis passed over resistor 106 to the base of another NPN type transistor107. This transistor is gated on to energize the first unit of coolingcapacity. By way of example, this signal can complete the energizingcircuit for the coil of a relay which puts the first of two compressorson the line, to initiate cooling of the controlled space. The circuit oftransistor 107 includes a diode 108 and a resistor 110, connected in amanner similar to diode 98 and resistor 100 in the circuit of transistor96.

' Considering now that the analog regulating signal on line 25 decreasesas the space is cooled by the first compressor coming on the line, thesignal level on common line 25 decreases and eventually falls below thelevel of the reference signal on line 48. At this time op amp 85switches and goes low, so that compressor number one is deenergized atthis time. If the analog signal on line 25 suddenly increases again, ina call for cooling, ordinarily the first compressor would immediately beswitched on by operation of cooling control stage 85 and transistor 107.To avoid rapid cycling of this compressor, an important feature of thisinvention is a provision of an off time-delay circuit including afield-effect transistor (FET) 111. Considering the circuit of FET 111,energizing conductor 36 is coupled over a resistor 112 to the source ofthis transistor, and three high-ohm resistors 113, 114 and 115 arecoupled in' series between conductor 36 and the upper plate of capacitor116. A table of actual circuit components is set out at the end of thespecification, but for the present it is sufficient to note that thesethree resistors provide a very high value of resistance betweenconductor 36 and capacitor 116. The lower plate of this capacitor iscoupled over conductors 117 and 118 to the output side of op amp 85,which must go high and provide a positive signal to bring on the firstcompressor again. Another capacitor 120 is coupled between conductor 36and a common circuit connection 121, with diode 122 coupled as shownbetween circuit point 121 and the source of PET 111. The drain ofthistransistor is grounded. Another diode 123 also provides a referenceconnection between the heating circuit and common circuit point 121,which is also coupled to the 2 input connection of op amp 85. v

The first call for cooling, which provided the high or positive outputsignal from op amp 85, was applied over conductors 118 and 117 to thelower plate of capacitor 116. This signal allowed the capacitor 116 tobe charged from conductor 36 over resistor 112, the source and gate ofFET 111, and resistor 124 to the upper plate of capacitor 116. Thusvirtually the entire B+ voltage, which was 24 volts positive on line 36in a preferred embodiment, is developed across capacitor 116 as thefirst compressor is brought on the line. As the cooling is initiallysatisfied and op amp 85 switches to a low output signal, this in effectprovides a ground or low level signal at the bottom plate of capacitor 116..

In effect this lower plate is now at minus B voltage, but the other sideof capacitor 116 is returned through a very high resistance (113-115) toline 36. Thus it takes an appreciable time, determined by the highresistance in circuit with capacitor 116, for this negative voltage onlines 117 and 118 to decrease to the point where the positive switchingof op amp 85 can override this voltage. In a preferred embodiment thetiming circuit including FET l1 1 was set to provide approximately a 3-minute off interval of compressor number one. With this circuit therepeated turn-on of the first compressor in the cooling system isprevented, saving undue wear which might otherwise be caused as theregulating signal on common line 25 changed back and forth at a levelvery close to the reference signal on line 48.

It is noted that transistor 130 was gated on as the positive signal fromop amp 85 was passed over line 118, resistor 132 and conductor 133 tothe base of transistor 130. Another resistor 134 is coupled between thebase and emitter of this transistor, and the diode 135 is con nected asshown between the collectorand emitter to protect transistor 130.Conduction of transistor 130 provides a signal on line 131 to energizethe hot gas controlling relay, or any other suitable capacity controlarrangement. This assumes a system in which the bypass of hot gas to thesuction side of the compressor is utilized.

As the level of the regulating signal on line 25 continues to increaseand eventually exceeds the level of the reference signal on line 46, opamp 86 will be switched to provide an output signal over line 125,resistor 126, and though resistor 127 to ground. Thus the voltagedeveloped across resistor 127 provides a signal to the gate oftransistor 128 which rapidly drives this transistor on and virtuallygrounds the base of transistor 130, to cut off this transistor anddeenergize the hot gas controlling relay or whatever other hot gascontrol component may be connected to conductor 131. In the circuit ofop amp 86, capacitor 136 is coupled between its input terminals. Inputconnection 6 receives a reference signal over resistor 137, and theother input connection is coupled over resistor 138 to common conductor25, to receive the analog regulating signal. Resistor 140 is coupledbetween output connection 7 and input connection 5, and diode 141 iscoupled between input connection 6 and conductor 142 to the on-timedelay circuit for the second compressor.

As the amplitude of the regulating signal on line 25 further increasesand exceeds the level of the signal on conductor 45, op amp 87 isswitched to provide a high output signal at its terminal 7. Thisamplifier stage has a capacitor 143 connected between its inputterminals, and input connection 6 receives the reference signal overresistor 144. The regulating signal on common line 25 is coupled overresistor 145 to the other input connection, which is also coupled overresistor 146 to output terminal 7. A diode 147 has its anode connectedto the common connection between resistors 145 and 146 and capacitor143, and its cathode coupled to conductor 118. As the output signalofterminal 7 of stage 87 goes high, this signal is extended over conductor148 to the emitter of a'PNP type transistor 150. However this circuitincludes another FET transistor 151 connected to provide a minimumon-delay, so that the second compressor cannot be energized by a signalof a step function type and an amplitude sufficient to bring oncompressors one and two at the same time.

In this second time-delay circuit the source of unit 151 is coupled overresistor 152 to conductor 148, and the gate of the transistor is coupledto the common connection between a very high resistance 153 and theupper plate of a capacitor 154, the lower plate of which is grounded.The upper end of resistor 153 is coupled to conductor 148. The drain ofPET 151 is coupled through a resistor 155 to ground. Another resistor156 is coupled between conductor 148 and the common connection betweenresistor 155 and the drain of FET 151. The source of this transistor iscoupled to the base of transistor 150, which has its collector coupledthrough a series circuit including resistors 157 and 158 to ground.Another NPN type transistor has its emitter grounded and its basecoupled to the common connection between resistors 157 and 158. Thecollector of this transistor 160 is coupled over line 161 to provideanother actuating output signal for bringing the second compressor onthe line, as by completing a relayenergizing circuit for thiscompressor. A diode 162 is coupled between the base and emitter oftransistor 160 forcircuit protection.

The resistances in the voltage divider circuit including FET 151 areselected so that the potential at the source of this unit is very closeto the full B voltagefor this circuit as received over conductor 148.Resistor 156 is sized electrically to have a value considerably lessthan the resistance of resistor 152. Accordingly the voltage at thedrain of PET 1511 is considerably lower than the voltage at its source.Thus even though the signal on line 48 is raised when cooling controlstage 87 is switched, the voltage applied to the base of PNP typetransistor 150 is not changed appreciably at this time, in that thevoltage at the drain of FET 151 remains close to ground. It is onlyafter capacitor 151 charges sufficiently to allow FET 151 to conduct,reducing the voltage at the base of transistor 150 and driving thistransistor on to provide gate drive for transistor 160, that the signalis supplied over line 161 to bring the second compressor on the line.Accordingly this circuit functions as an inverse of the timing circuitincluding FET 111. This on-time delay circuit prevents simultaneousenergization of the compressors to obviate a large drain on theelectrical supply, which could blow a fuse or even damage equipment ofthe motorgenerator type which has a limited electrical capacity.

The last cooling stage including op amp 88 is operated when the level ofthe regulating signal on common line 25 passed over resistor 163 to the3 input connection of this op amp exceeds the level of the referencesignal passed over conductor 44 and resistor 164 to the 2 inputconnection. A capacitor 165 is coupled between the input connections ofthis stage, and a resistor 166 is coupled between output connection 1and input connection 3. Thus as this stage switches and provides a highoutput signal on conductor 167, the actuating output signal is passedover resistor 168 to the common connection between resistors 126 and127, to regulate the hot gas or other capacity control arrangement.

From this explanation it will be apparent that there is a sequentialoperation of the cooling control stages 85-88 and the associated coolingequipment as the amplitude of the analog regulating signal on commonline 25 gradually increases when the system calls for additionalcooling. From the set point established by unit 14, when the temperaturein the control space rises 75, op amp 85 switches to bring the firstcompressor on the line and, through transistor 130, to bring the hot gason. With an additional half a degree rise to one degree above the setpoint, op amp 86 is switched high to drive transistor 128 on and rob thedrive from transistor 130, turning the hot gas off. With an additionalhalf a degree temperature rise to a value 1.5 degrees F. above the setpoint, op amp 87 is switched on to initiate the timing interval so thatafter FET 151 conducts, transistors 150 and 160 are driven on to bringthe second compressor on the line. As soon as op amp 87 conducts, asignal is extended from output pin 7 over resistor 146, diode 147, andresistor 132 to energize transistor 130 and bring the hot gas relay on.With a further increase in temperature of half a degree, or a total oftwo degrees F. above the set point, op amp 88 is switched to provide asignal over line 167 and resistor 168 to drive on transistor 128, turnoff transistor 130, and shut off the hot gas. With a reversal of thetemperature change in the controlled space, the same functions occur inthe opposite sequence. The value of the reference signal on line 49 isestablished so that as op amp 90 is switched, and the heat brought on,when the temperature in the controlled space is one degree F. below thatestablished by the temperature set unit 14.

To enable those skilled in the art to practice the invention, a table ofcircuit component identifications and values is set out below. These aregiven solely to enable those skilled in the art to practice theinvention with a minimum of experimentation, and in no way rep resent aconstraint upon the concept or implementation of the invention. Thecircuit described and illustrated was operated with a d-c potential of24 volts positive on conductor 36 with respect to ground. The integratedcircuits shown, including the units 21, 30, 56, 85-88 and 90, were allof the Signetics type N555 8V. The transistors, other than thoseidentified below, were 2N34l6 and the diodes were BAX 1 8. In generalthe resistors were one-half watt, percent, unless otherwise specified.

Component Identification or Value 111,151 2N3822 150 2N5365 10,11Fenwall UUA33J1 76 1N5247-B 59,82,93,102 0.1 at, 25 V.

136,143,165 0.1 pf, 25 V. 77,116,154 2 pf, 200 V. 91,120 50 pf, 16 v.

40,42 100 41 200 43,65,l00,110,127,l34,158 l K 52 ZlO 741W, 1% 53 32.4AW, 1% 54 26] riaW, 1% l0 62,113,114,115,153 20 M 66,70,92,101,103,137100 K 138,144,145,163,l64 100 K 71 10 K w, 1% 72,73 62 K 74 51078,94,104,140,146,166 4.7 M 15 83 10 M 95,106,112,124,126 7.5 Kl32,152,155,157,168 7.5 K

While only a particular embodiment of the invention has been describedand illustrated, it is manifest that various modifications andalterations may be made therein. It is therefore the intention in theappended claims to cover all such modifications and alterations as mayfall within the true spirit and scope of the invention.

What is claimed is:

l. A control arrangement for regulating operation of a cooling systemwhich discharges air into a space and receives return air from thespace, which control arrangement comprises:

a first thermostat disposed to provide a temperature signal indicativeof the discharge air temperature;

a second thermostat, disposed to provide a second temperature signalindicative of the return air temperature;

a temperature set unit, adjustable to provide a set signal indicatingthe desired temperature of the space;

a selection circuit, connected to receive both the set signal and afirst reference signal, which selection circuit completes an operatingcircuit for the first thermostat when the value of the set signal isless than the value of the first reference signal, and completes anoperating circuit for the second thermostat when the value of the setsignal is greater than the value of the first reference signal; and

a comparator circuit, connected to receive both the set signal and thetemperature signal from the energized one of the thermostats, and toprovide a regulating signal, related to the difference between the setsignal and the temperature signal from the energized thermostat, toregulate associated equipment.

2. A control arrangement as claimed in claim 1, and further comprising aheat override circuit, connected to receive both the set signal and asecond reference signal, operative to provide a lock-out signal toprevent operation of associated heating equipment when the temperaturecalled for by the set signalis below the value of the temperaturerepresented by the second reference signal.

3. A control arrangement for regulating operation of a cooling systemwhich discharges air into a space and receives return air from thespace, which control arrangement comprises:

a first Thermistor disposed to provide a first tempe rature signalindicative of the discharge air temperature;

a second Thermistor, disposedto provide a second temperature signalindicative of the return air temperature;

a temperature set potentiometer, adjustable to provide a set signalindicating the desired temperature of the space;

a selection circuit, including a voltage level detector connected toreceive both the set signal and a first reference signal, a firsttransistor connected to complete an operating circuit for the firstThermistor when the value of the set signal is less than the value ofthe first reference signal, and a second transistor connected tocomplete an operating circuit for the second Thermistor when the valueof the set signal is greater than the value of the first referencesignal; and

a comparator circuit, connected to receive both the set signal and thetemperature signal from the energized one of the Thermistors, and toprovide a regulating signal, related to the difference between the setsignal and the temperature signal from the energized Thermistor, toregulate associated equipment.

4. A control arrangement for regulating operation of a cooling systemwhich discharges air into a space and receives return air from the samespace, including first and second thermostats disposed to providetemperature signals indicating the discharge and return air temperature,a temperature set unit providing a set signal to indicate the desiredspace temperature, a selection circuit completing an operating circuittoonly one of the thermostats as a function of the set signal and a firstreference signal, a comparator circuit receiving the set signal and thetemperature signal from the energized thermostat to provide on a commonline a regulating signal which is a function of the difference betweenthe set signal and the actual temperature signal from the energizedthermostat, and at least two cooling control stages, each having aninput portion coupled to the common line, and each having an outputcircuit individually coupled to different cooling components, to bringon one of the different cooling components at different amplitude levelsof the regulating signal on the common line.

5. A control arrangement for regulating operation of a cooling system bycomparing a temperature set signal with an actual temperature signal andproviding a regulating signal, related to the difference between the setsignal and the actual temperature signal to regulate cooling equipmentincluding at least two compressors through a plurality of coolingcontrol stages, which control arrangement comprises:

a common line connected to receive the regulating signal;

a first cooling control stage, having a first input connection coupledto the common line and a second input connection connected to receive afirst reference signal, to provide an actuating output signal when theregulating signal exceeds the first reference signal and bring the firstcompressor on the line;

a second cooling control stage, having a first input connection coupledto the common line and a second input connection coupled to receive asecond reference signal, to provide an actuating output signal when theregulating signal exceeds the second reference signal and bring thesecond compressor on the line; and

a first time-delay circuit, coupled to said first cooling control stage,effective to maintain said first controlling stage in the off conditionfor a predetermined minimum time interval after the regulating signal onthe common line has decreased below the value of the first referencesignal, to avoid excess cycling of the first compressor.

6. A control arrangement as claimed in claim 5, and

further comprising a second time-delay circuit, coupled to said secondcooling control stage, effective to prevent said second cooling controlstage from providing

1. A control arrangement for regulating operation of a cooling systemwhich discharges air into a space and receives return air from thespace, which control arrangement comprises: a first thermostat disposedto provide a first temperature signal indicative of the discharge airtemperature; a second thermostat, disposed to provide a secondtemperature signal indicative of the return air temperature; atemperature set unit, adjustable to provide a set signal indicating thedesired temperature of the space; a selection circuit, connected toreceive both the set signal and a first reference signal, whichselection circuit completes an operating circuit for the firstthermostat when the value of the set signal is less than the value ofthe first reference signal, and completes an operating circuit for thesecond thermostat when the value of the set signal is greater than thevalue of the first reference signal; and a comparator circuit, connectedto receive both the set signal and the temperature signal from theenergized one of the thermostats, and to provide a regulating signal,related to the difference between the set signal and the temperaturesignal from the energized thermostat, to regulate associated equipment.2. A control arrangement as claimed in claim 1, and further comprising aheat override circuit, connected to receive both the set signal and asecond reference signal, operative to provide a lock-out signal toprevent operation of associated heating equipment when the temperaturecalled for by the set signal is below the value of the temperaturerepresented by the second reference signal.
 3. A control arrangement forregulating operation of a cooling system which discharges air into aspace and receives return air from the space, which control arrangementcomprises: a first Thermistor disposed to provide a first temperaturesignal indicative of the discharge air temperature; a second Thermistor,disposed to provide a second temperature signal indicative of the returnair temperature; a temperature set potentiometer, adjustable to providea set signal indicating the desired temperature of the space; aselection circuit, including a voltage level detector connected toreceive both the set signal and a first reference signal, a firsttransistor connected to complete an operating circuit for the firstThermistor when the value of the set signal is less than the value ofthe first reference signal, and a second transistor connected tocomplete an operating circuit for the second Thermistor when the valueof the set signal is greater than the value of the first referencesignal; and a comparator circuit, connected to receive both the setsignal and the temperature signal from the energized one of theThermistors, and to provide a regulating signal, related to thedifference between the set signal and the temperature signal from theenergized Thermistor, to regulate associated equipment.
 4. A controlarrangement for regulating operation of a cooling system whichdischarges air into a space and receives return air from the same space,including first and second thermostats disposed to provide temperaturesignals indicating the discharge and return air temperature, atemperature set unit providing a set signal to indicate the desiredspace temperature, a selection circuit completing an operating circuitto only one of the thermostats as a function of the set signal and afirst reference signal, a comparator circuit receiving the set signaland the temperature signal from the energized thermostat to provide on acommon line a regulating signal which is a function of the differencebetween the set signal and the actual temperature signal from theenergized thermostat, and at least two cooling control stages, eachhaving an input portion coupled to the common line, and each having anoutput circuit individually coupled to different cooling components, tobring on one of the different cooling components at different amplitudelevels of the regulating signal on the common line.
 5. A controlarrangement for regulating operation of a cooling system by comparing atemperature set signal with an actual temperature signal and providing aregulating signal, related to the difference between the set signal andthe actual temperature signal to regulate cooling equipment including atleast two compressors through a plurality of cooling control stages,which control arrangement comprises: a common line connected to receivethe regulating signal; a first cooling control stage, having a firstinput connection coupled to the common line and a second inputconnection connected to receive a first reference signal, to provide anactuating output signal when the regulating signal exceeds the firstreference signal and bring the first compressor on the line; a secondcooling control stage, having a first input connection coupled to thecommon line and a second input connection coupled to receive a secondreference signal, to provide an actuating output signal when theregulating signal exceeds the second reference signal and bring thesecond compressor on the line; and a first time-delay circuit, coupledto said first cooling control stage, effective to maintain said firstcontrolling stage in the off condition for a predetermined minimum timeinterval after the regulating signal on the common line has decreasedbelow the value of the first reference signal, to avoid excess cyclingof the first compressor.
 6. A control arrangement as claimed in claim 5,and further comprising a second time-delay circuit, coupled to saidsecond cooling control stage, effective to prevent said second coolingcontrol stage from providing an actuaTing output signal for apredetermined minimum time interval after the regulating signal on thecommon line exceeds the value of the second reference signal, to preventsimultaneous energization of the first and second compressors andobviate a large drain on the power supply.